Batteries which use lithium-ion or nickel-metal hydride technology and have a large number of electrochemical battery cells connected in series are used nowadays, especially in hybrid and electric vehicles. A battery management unit is used to monitor the battery and, in addition to monitoring safety, is intended to provide the longest possible service life. For this purpose, the voltage of each individual battery cell is measured together with the battery current and the battery temperature and the state is estimated (for example the state of charge or the aging state of the battery). In order to maximize the service life, it helps to know the currently given maximum capacity of the battery, that is to say the maximum electrical power which can be delivered or consumed, at any time. If this capacity is exceeded, the aging of the battery can be greatly accelerated.
In order to make it possible to accurately measure the voltage of each individual battery cell or at least the voltage of each battery module which comprises a predetermined number of battery cells, battery management units are known from the prior art, which units comprise a plurality of series-connected monitoring chips, that is to say integrated circuits, which can carry out voltage measurements, among other things, and are connected in the form of a daisy chain to a first bus which enables communication between the individual monitoring units without requiring DC isolation or the use of high-voltage electronics. In this context, the monitoring units are located with their supply voltages, which are delivered by the battery cells or battery modules to be monitored, in a voltage chain and communicate with one another in such a manner that each monitoring unit communicates only with an adjacent monitoring unit and forwards the communication data which come from monitoring units, which have a higher voltage level, to the monitoring unit which is in each case lower in terms of the voltage level.
A base monitoring unit which is likewise connected to the first communication bus and can receive messages from each of the monitoring units is arranged at that end of the communication bus which is lowest with regard to the voltage level. In addition, the base monitoring unit is connected via a second bus to a control device which receives the forwarded data via this bus. DC isolation is usually provided between the base monitoring unit and the control device.
The monitoring units are usually placed in the vicinity of the battery modules associated with them and the connections for communication via the first bus and the second bus are implemented by installing cable harnesses.
Application-specific integrated circuits (ASIC) which can be configured in accordance with the modular principle either as a base monitoring unit at the lower end of the voltage chain—that is to say with an interface for communicating with the control device and an interface for communicating with a further monitoring unit in a voltage chain—or also as a monitoring unit in the daisy chain are known from the prior art.
Whereas the first bus which is used for first communication between the individual monitoring units is usually designed for reduced EMC (electromagnetic compatibility) emission and an increased resistance to irradiation, the second bus which is used for communication with the control device is usually a standardized and non-proprietary bus which is suitable for communication with a microcontroller included in the control device. In this case, use is often made of a protocol which, from the point of view of electromagnetic compatibility, is not intended to be routed over a relatively long distance via a cable.